Fecal occult blood testing, also known as FOBT, is a test that can detect blood in the stool, even when it is not visible to the eye. This test involves placing a small amount of stool on a special card. A solution (or developer) is then applied to the card, and the card turns blue if blood is present. This test can be done at your doctor's office, or at home, and brought to the doctor's office for developing. Polyps or colon and rectal tumors may cause intermittent bleeding, and FOBT may be able to detect this bleeding, prompting further testing. It is important to note that some foods may cause the test to be positive. Ask your physician what foods to avoid when doing this test. The American Cancer Society recommends that this test be done every year, in conjunction with a sigmoidoscopy every 5 years.